Method of electrodepositing a metallic coating



United States Patent Ofiice 2,727,856 Patented Dec. 20, 1955 P rl METHOD OF ELECTRODEPOSITING A METALLIC CGATKNG John G. Beach, Coiumhus, Ohio, assignor to the United States of America as represented by the Secretary of the Army No Drawin". Application April 3, 1952, Serial No. 280,367

2 Claims. (Cl. 204-43) This invention relates to lead-zinc alloys, to a process of plating lead-zinc alloys, to an electrolyte useful in the process for producing lead-zinc alloys and an article having a lead-zinc alloy thereon. More particularly, it relates to a process for the co-electrodeposition of lead and zinc to form a lead-zinc alloy.

Lead-zinc alloys make very effective coatings to replace tin on copper wire. Inasmuch as tin is a critical material in a time of war or extensive military preparedness, a lead-zinc alloy could be of great technical advantage. In addition, since lead-zinc alloys have a high fusing temperature, they may be quite useful as coatings on copper wire in situations where it is desired to use a solder with a higher melting point than the conventional tin-lead solders.

Since the solubility of zinc in molten lead, at the dipping temperature, is only about 1.5% this method cannot be used to prepare lead-zinc alloys containin more than 1.5% zinc. The electrolyte heretofore used for electroplating lead-zinc alloys has been an electrolytic bath containing lead fluoborate, zinc fluoborate and ammonium fluoborate. However, this bath is very unsatisfactory when used to co-electrodeposit lead and zinc as a leadzinc alloy on copper wire since little or no zinc is obtained in the deposits.

It is therefore a primary object of the present invention to provide an electrodeposited lead-zinc alloy containing substantially from .5 t 3% Zinc, such alloy being adherent to copper wire.

It is a further object to provide a process for co-eiectrodepositing a lead-zinc alloy and having the aforementioned improved qualities.

Another object is to provide an electrolytic bath capable of producing a lead-zinc alloy having the aforementioned improved qualities.

These and other objects and advantages of the present invention will be better understood as the detailed descrip tion thereof progresses.

In accordance with the present invention, there is provided a wire conductor having a coating thereon, said coating comprising an electrodeposited alloy comprising lead and zinc, a preferred embodiment of this coating being one wherein the zinc comprises 0.5% to 4% by weight, the balance being lead.

Further, in accordance with the present invention, there is provided a method of electrodepositing a lead-Zia: alloy upon a wire conductor comprising immersing the article to be plated in an aqueous electrolyte containing zinc cyanide, a hydroxide selected from the group consisting of sodium, potassium and ammonium hydroxid a cyanide selected from the group consisting of sodium potassium and ammonium cyanides, basic lead acetate, a fluoride selected from the group consisting of sodium, potassium and ammonium fluorides, potassium sodium tartrate, gum arabic and a betaine, and passing an electric current through the electrolyte in such a manner that the article to be plated becomes a cathode.

Also, in accordance with the present invention there is provided an electrolyte for co-electrodepositing a leadzinc alloy comprising in aqueous media, zinc cyanide, a hydroxide selected from the group consisting of sodium, potassium and ammonium hydroxides, a cyanide selected from the group consisting of sodium, potassium and ammonium cyanides, basic lead acetate, potassium sodium tartrate, a fluoride selected from the group consisting of sodium, potassium and ammonium fluorides, gum arabic and a betaine. A preferred embodiment of the electrolyte comprises zinc cyanide in a concentration up to about 100 grams per liter, to grams per liter of sodium hydroxide, 35 to 40 grams per liter of sodium cyanide, 3 grams per liter of basic lead acetate, 25 to 30 grams per liter of sodium tartrate, 4 to 6 grams per liter of sodium fluoride, 3 grams per liter of gum arabic and 10 to 20 cc. per liter of a betaine.

It has been found from the present invention that lead and zinc can be co-electrodeposited from an electrolytic bath to form an adherent alloy on copper wire. It has been found that this alloy may be deposited on a cathode from a strongly alkaline aqueous bath which is operated at relatively low current densities and temperatures.

The alloy as deposited from the above electrolytic bath is essentially a pure bright alloy of electrodeposited zinc and lead. Wherever, due to a change in the amount of bath constituents or in operating conditions, an increase in the Zinc content of the alloy occurs, there will be a corresponding decrease in the lead content so that there are substantially, only two elements present, zinc and lead. Likewise, Where there is a decrease in the zinc content, there will be an increase in the lead content of the plate.

To deposit this alloy by electrodeposition on copper wire, the following method and materials are used.

The anodes consist of pure lead anodes of an inch in diameter and 6 inches in active length. Eight anodes are used and they are spaced four in a row, their axes being /8 of an inch apart, the two rows being placed 4% inches apart.

The cathodes are constructed by taking sixteen gauge copper wire (0.05082 inch diameter) and straightening it by stretching a 33 inch length to 36 inches by means of a vise and a pair of pliers, cutting the straightened wire to lengths to provide six inches in active cathode length, and racking 5 lengths of such wire so they lie in a vertical plane with axes 0.75 inch apart.

Prior to the electroplating process, the copper wires forming the cathode are subjected to a preliminary treatment. This consists of vapor degreasing; electrochemically cleaning them in a bath composition of 10 liters volume containing a concentration 75 grams of Anodex per liter wherein the copper wires are made the anode and a current of density of 90 amperes per square foot is passed through the solution for one minute. the solution being held at 200 F. (93 C.) during the electrochemical cleaning; cold rinsing the copper wires; acid dipping them for one minute in a 10 liter acid bath held at F., said bath consisting of 0.5 normal hydrochloric acid and ammonium chloride in a concentration of grams per liter; and again cold rinsing the copper wires.

To electroplate the lead and zinc, the aforementioned cathode is immersed in an electrolytic bath of 2.8 liters containing 90 grams per liter of zinc cyanide, 96 grams per liter of sodium hydroxide, 37.5 grams per liter of sodium cyanide, 2.5 grams per liter of basic lead acetate, 27.7 grams per liter of potassium sodium tartrate, 5.3 grams per liter of sodium fluoride, 2.7 grams per liter of gum arabic and 15.5 cubic centimeters per liter of betaine such as RH 556 (Du Pont product). The bath is heated to a temperature of about 50 C. and a current with a density of about 3 amperes per square foot is passed through the solution. During the passage of the current the cathode is agitated by work-rod movement at 3 the rate of about 66 strokes per minute, each stroke being 34 of an inch in length. To obtain a coating with a thickness of 0.00001 inch, the plating time required is 90 seconds and a thickness of 0.00006 inch requires 540 seconds. H

A 0.00006' inch" thickness of this alloy coating when tested for solderability showed a solderability superior to a tin coating of equal thickness; The test consisted of measuring the capillary'rise of 50 lead-50 tin solder liquid along two twisted pairs of wires. Where coating of leadzinc alloy showed a rise of inch, a standard tin coating showed a rise of only inch.

The coating was tested for adhesion by bending coated wire specimens around a rod having a diameter equal to four times the diameter of the wire, anddipping the bent portion of the wire in a' sodium polysulphide solution for 30 seconds. When subjected to this test there was no blackening of exposed copper. Blackening would result in a situation where there was cracking or parting of the coating.

While there have been described what at present are considered to be preferred embodiments of the invention, it will be understood by those skilled in the art that various changes and modifications may be made herein without departing from the invention and it is therefore aimed in the appended claims to-cover all such modifications as fall within the spirit and scope of the invention.

What is claimed is:

1. In the method of co-electrodepositing a bright leadzinc alloy on a wire conductor and containing from 0.5% to 4% by weight of zinc and the balance lead, the steps comprising immersing the wire conductor to be coated in an electrolytic bath consisting essentially of an aqueous solution containing up to about 100 grams per liter of zinc cyanide, 75 to 100 grams per liter of sodium hydrox ide, to grams per liter of sodium cyanide, 3 grams per liter of basic lead acetate, 25 to 30 grams per liter of potassium sodium tartrate, 4 to 6 grams per liter of sodium fluoride, about 3 grams per liter of gum arabic and 10 to 20 cubic centimeters per liter of a betaine; passing an electric current through the bath in such a manner that the wire conductor to be plated becomes a cathode; regulating the electric current to provide a current density of about 3 amperes per square foot; and heating the bath to a temperature of about C.

2. Au electrolytic bath for co-electrodepositing a bright lead-zinc alloy containing 0.5% to 4% by weight of zinc and the balance lead comprising an aqueous solution containing substantially up to 100 grams per liter of zinc cyanide; to grams per liter of sodium hydroxide; 35 to 40 grams per liter of sodium cyanide, 3 grams per liter of basic lead acetate, 25 to 30 grams per liter of potassium sodium tartrate, 4 to 6 grams per liter of sodium fluoride, 3 grams per liter of gum arabic and 10 to 20 cubic centimeters per liter of a betaine.

References Cited in the file of this patent UNITED STATES PATENTS 897,593 Allen Sept. 8, 1908 1,464,506 Goff Aug. 14, 1923 1,509,102 Dana Sept. 23, 1924 2,184,179 Domrn Dec. 19, 1939 2,445,858 Mitchell et a1 July 27, 1948 2,446,716 Nachtman Aug. 10, 1948 2,472,296 Hartnell June 7, 1949 2,474,092 Liger June 21, 1949 2,545,566 Bone Mar. 20, 1951 OTHER REFERENCES Metal Industry, June 25, 1943, pp. 407411. 

1. IN THE METHOD OF CO-ELECTRODEPOSITING A BRIGHT LEADZINC ALLOY ON A WIRE CONDUCTOR AND CONTAINING FROM 0.5% TO 4% BY WEIGHT OF ZINC AND THE BALANCE LEAD, THE STEPS COMPRISING IMMERSING THE WIRE CONDUCTOR TO BE COATED IN AN ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING UP TO 100 GRAMS PER LITER OF ZINC CYANIDE, 75 TO 100 GRAMS PER LITER OF SODIUM HYDROXIDE, 35 TO 40 GRAMS PER LITER OF SODIUM CYANIDE, 3 GRAM PER LITER OF BASIC LEAD ACETATE, 25 TO 30 GRAMS PER LITER OF POTASSIUM SODIUM TARTRATE, 4 TO 6 GRAMS OF SODIUM FLUORIDE, ABOUT 3 GRAMS PER LITER OF GUM ARABIC AND 10 TO 20 CUBIC CENTIMETERS PER LITER OF A BETAINE; PASSING AN ELECTRIC CURRENT THROUGH THE BATH IN SUCH A MANNER THAT THE WIRE CONDUCTOR TO BE PLATED BECOMES A CATHODE; REGULATING THE ELECTRIC CURRENT TO PROVIDE A CURRENT DENSITY OF ABOUT 3 AMPERES PER SQUARE FOOT; HEATING THE BATH TO A TEMPERATURE OF ABOUT 50* C. 